Make filter scaling geometric

According to section 4.1 of
http://www.sciencedirect.com/science/article/pii/S0020019006003127, we
can and should scale the size of the filters (orthogonally to the
requirements for false positive probability), by a factor greater than
1. This adds scaling by a factor of 2--each additional filter can handle
twice the number of elements of the previous one.